Interesting thoughts and observations on OD transmissions

TrailBeast said
In my specific situation I have a 42RH Dodge transmission in my Dart, and have had it in there for 6 or so years and it has been flawless.

Recently I have an issue where it doesn’t stay in converter lockup consistently.
It will drop out and re engage randomly.
My setup is fully automatic using governor pressure to determine shift points using adjustable pressure switches.
The lockup normally engages at about 65mph, and disengages when under 65 if the switch inside the cabin is in the Auto position.

I used a jumper wire to bypass the adjustable pressure switch to start diagnosing the intermittent on/off of converter lockup, then go to the relay that actually switches it if needed.
The question was, is the problem the pressure switch, the relay, or the solenoid inside the trans?
Well, bypassing the pressure switch solved the problem and in that process made lockup totally manual depending on the switch inside the car. (Which I expected)
Of course the problem is the most expensive part of the shifting system.
Those pressure switches ain't cheep.
I hardly use lockup except for extended 80mph and up drives, but it would be nice to have it all working as designed.

Anyway, here’s the interesting part.
I knew OD only works off the 3rd gear oil, meaning OD won’t engage unless the trans is already in 3rd gear.
Well during the lockup diagnosis I also found lockup will only engage once in 3rd.
With the pressure switch bypassed I can manually engage lockup at any speed I want as long as the trans is already in 3rd gear.
(Kinda fun to play with, but no practical use)

Now here’s the actual interesting thought based on how this trans works.
Imagine doing 80+ in OD with converter lockup and manually downshifting to second gear.
Converter lockup, OD and 3rd gear will all drop out at the same time.
Depending on the rear gears this could be a real problem.
I have a ratshit shifter, so this could easily happen accidentally.

Interesting.

I find it interesting that you have your OD set up to lock so late. What differential gear are you running?

I had that same type of pressure switch setup in my car with a 46rh. I had the pressure switches adjusted for "normal" acceleration, so it went into OD at about 50 mph and lockup at about 55 mph (3.23 gear, 26" tire). Of course, if I was going to do any sort of spirited acceleration I would have to manually shut off OD and LU ahead of time, otherwise I would be shifting to OD and locking up as soon as it shifted into 3rd gear, which could be way too early.

But mine wouldn't unlock at those same speeds. It unlocked when I hit about 50 mph (about 1500 rpm), and it wouldn't drop out of OD until I hit about 45 mph (about 1300 rpm). I guess that's just how my pressure switches worked; the pressure to close the circuit was X and the pressure to open the circuit was X minus a bit.

I'm running factory Magnum EFI, so I eventually got rid of the pressure switches and wired the computer to control the OD and LU. I was pretty surprised how quickly the computer would tell it to go into OD and LU during light acceleration. During light acceleration 3rd only lasts about 1 second, it will shift into OD almost immediately and put the RPMs at about 1200. If I keep accelerating gently or start cruising it will lockup and end up at about 1300 rpm (about 45 mph). I thought that was super low and there must be a problem, then I found this in the 1995 service manual: